Interpretation of results on petrosal sinus samples assayed for pituitary hormones, particularly ACTH, depends upon a knowledge of normal hormonal concentrations in these vessels. Such data is not available. Our experience thus far has been restricted to patients with hormonally-active pituitary tumors, principally ACTH-producing, but a few GH, TSH and prolactin-secreting tumors. Our small experience in patients with ectopic ACTH syndrome does not provide an adequate normal cohort. Experience with sampling for non ACTH-secreting endocrine tumors suggests that hormone concentration gradients are found in normal subjects and depend upon the anatomy of the draining veins and the proximity of the catheter tip to the functioning gland. To interpret reliably the results of petrosal sinus sampling, particularly in patients in whom the diagnosis of Cushing's syndrome is questionable, a detailed knowledge of the magnitude of lateralizing ACTH gradients in normal subjects is required. In 20 patients undergoing parathyroid venous sampling for localization of occult parathyroid adenomas, we will advance the catheters into the inferior petrosal sinuses, after superior thyroid venous sampling, to obtain blood samples for determination of ACTH and other pituitary hormones, before and after the infusion of corticotropin releasing hormone (CRH).